Known humidity sensors of the type that detects the relative humidity of air, by absorbing water from the ambient into a polymer, where the principle of measuring is, to detect the change of the dielectric constant or conductivity of this polymer as a function of the relative humidity, has the implicit weakness, that they deteriorate over time, if they frequently are exposed to condensing conditions, moisture and dirt. As a consequence of this, most humidity sensors are equipped with some kind of protective encapsulation.
Humidity sensors of this type are nevertheless unsuited for use under conditions with condensation and moisture. The reason for this is, that the dead-space volume may be several thousand mm3, allowing the amount of water condensing inside the encapsulation dead-space volume to accumulate and get so large, that it becomes a significant problem because of the irreversible damage it may cause on the sensing element.
The large amount of condensed water takes very long time to evaporate and diffuse out of the encapsulation. This is a major disadvantage for using this kind of humidity sensor in wet and condensing environment.